Optical fibers are long, thin waveguide strands. They are used in a variety of applications involving the reception, transmission and detection of radiation, ranging from optical telecommunications to scientific instrumentation to ornamental displays. Conventional optical fibers have a high-index core surrounded by a low-index cladding, a geometry that results in the total internal reflection of radiation entering the fiber over a select range of angles. Other forms of conventional optical fibers are referred to as “light tunnels” and use reflection from reflective walls rather than total internal reflection. Conventional optical fibers can have a variety of core geometries, including elliptical cores, double cores, polarization-maintaining cores, and hollow cores.
Hollow core optical fibers are desirable where transmission losses need to be minimized, since in conventional optical fibers most of the radiation loss in an optical fiber occurs from absorption in the solid core. However, conventional optical fibers that depend on total internal reflection for waveguiding and confinement need to have a core with a higher dielectric constant than that of the cladding. Such a structure is at odds with an optical fiber having a hollow central core, since air has a dielectric constant lower than all known practical cladding materials. Thus, hollow core optical fibers generally require specialized design considerations.
There are a number of different types of hollow core optical waveguides that operate on the either principle of total internal reflection or the principles of conventional reflection. For example, hollow optical waveguides through silicon wafers with highly reflective coatings on the side of the hollow core are disclosed in U.S. Pat. No. 6,090,636 to Geusic et al. An optical fiber interconnect through a silicon wafer with a hole filled with two different dielectric materials is disclosed in U.S. Pat. No. 6,150,188 to Geusic et al. A hollow optical fiber or hollow core waveguide consisting of a dielectric material coated with a highly reflective exterior coating is disclosed in U.S. Pat. No. 5,815,627 to Harrington, wherein the hollow core can contain one guided wave and the annular ring another guided wave. A hollow core optical waveguide having a highly reflective coating on the inside of a glass tube is disclosed in U.S. Pat. No. 6,141,476 to Matsuura.
As mentioned above, a desirable property for an optical fiber is minimal transmission loss. Accordingly, there has been significant effort to form optical fibers from materials that have high transmission and high reflection. Some of this effort has been directed to forming optical fibers from quasi-two-dimensional photonic crystals. A photonic crystal is a substrate within which is formed an array of period structures through which radiation of a particular wavelength or energy is forbidden to propagate. The result is a material with a very high reflectivity. Two-dimensional photonic bandgap crystal structures have been reported not only at optical wavelengths but at acoustic wavelengths as well.
It is well known in the semiconductor industry that a series of cylindrical holes judiciously formed in a solid semiconductor material can be used to form a quasi-two-dimensional photonic bandgap crystal. FIG. 1 is a plan view of a conventional two-dimensional photonic crystal formed from a substrate 110 patterned with an triangular array of cylindrical holes 120.
An example prior art quasi-two-dimensional photonic bandgap optical fiber 210 is illustrated in FIGS. 2A and 2B. The optical fiber 210 includes an array of holes 220 formed within a cylindrical substrate 226. The fiber includes an outer cladding 228. A solid core 232, referred to as a “core defect,” is at the center of the substrate. Radiation 242 is reflected within the bandgap of the photonic crystal produced by the periodic array of holes surrounding the core and is confined to the core. The radiation travels down the length of the solid core by total internal reflection made possible by the low average index of refraction of the cladding as compared to that of solid core 232. Though optical fiber 210 is photonic crystal based, it utilizes total internal reflection like a conventional optical fiber and is relatively lossy because the solid core absorbs radiation.
Accordingly, what is needed is a photonic crystal optical fiber with a hollow core that allows for low-loss radiation propagation down the fiber by virtue of highly reflectivity walls as a result of the photonic bandgap of the crystal.